Corrosion inhibition

ABSTRACT

An improved method and apparatus for protecting refinery equipment subject to condensate corrosion. Neutralizing inhibitors, such as morpholine, are added to refinery systems based on the pH of the initial condensate which is determined by using a special heat exchange technique.

United States Patent Sawyer [54] CORROSION INHIBITION [72] Inventor: Roy D. Sawyer, Toledo, Ohio [73] Assignee: Nalco Chemical Company, Chicago, Ill.

[22] Filed: Mar. 3, I970 21 App1.No.: 16,058

[52] US. Cl. ..2I/2.5, 21/2.7, 23/230 A, 73/4215 R, 73/422 R, 203/7, 208/47 [51] Int. Cl. ..C23f 14/02 [58] Field of Search ..2l/2.5, 2.7; 23/230 A, 230, 23/230 C; 203/7; 208/47 [56] References Cited UNITED STATES PATENTS 2,582,138 1/1952 Lane et al. ..2l/2.7 X 2,607,718 8/1952 Suthard 3,078,223 2/1963 3,088,795 5/1963 3,361,150 l/l968 6/1969 Crawford ..21/2.5

[ 1 Mar. 14, 1972 OTHER PUBLICATIONS Ristroph et al., Controlling lron & Copper Pickup with Neutralizing Amines Trans. of the ASME, Feb. 1956; pp. 287- 293 Sindery, G. G.; Corrosioan & Water Treatment ;Corrosion Technology; Proc. of Corrosion Convention, London, 1957; Leonard Hill House; London; 1958; pp. 20- 34 Sperry, S. M.; Reduction of Iron & Copper Corrosion in Steam & Water Cycles with Amines" Combustion; Nov. 1955;Pp. 65-71 Primary Examiner-Morris O. Wolk Assistant ExaminerBarry S. Richman AttorneyJohn G. Premo, Charles W. Connors and Edward A. Ptacek [57] ABSTRACT An improved method and apparatus for protecting refinery equipment subject to condensate corrosion. Neutralizing inhibitors, such as morpholine, are added to refinery systems based on the pH of the initial condensate which is determined by using a special heat exchange technique.

4 Claims, 1 Drawing Figure -INITIAL COHDENSATE SAMPLE COLLECTOR- COOLINQ WATER HYDROCARBON VAPOR OUT COOLING WATER HYDRO CARGO N VAPOR 24 TNERMOMETER 34 WELL GLASS SAMPLE COLLECTOR CORROSION INHIBITION INTRODUCTION The present invention is directed to the prevention or control of corrosion of oil refining equipment. More particularly, the subject invention is directed to a process for eliminating acid corrosion which takes place at the point of initial water condensation in petroleum distillation units.

Petroleum crudcs as well as gas oil, reduced crude, etc., are subjected to various processes in order to form lower boiling components such as gasoline. The product that is obtained from conversion is distilled to produce a gasoline fraction, a fuel oil fraction, lubricating oil fraction, etc. The lower boiling fractions and particularly gasoline are recovered as an overhead fraction from the distilling zones. The intermediate components are recovered as side cuts from the distillation zone. The fractions are cooled, condensed, and sent to collecting equipment. No matter what the source of the oil that is subject to distillation it has been found that corrosion of the equipment takes place. Acidic materials that are present in all crudes are carried along from the distillation zone with the distillate product and often cause extensive corrosion to take place on the metal surfaces of fractionating towers such as crude towers, trays within such towers, heat exchangers, receiving tanks, connecting pipes, etc. The most serious corrosion occurs in condensers and in the overhead line leading from the fractionating towers. The overhead line is used as a connection between the distillation tower and condensers. The distillate or stock which will be stored or used subsequently to charge other refining processes is condensed on the cooled surfaces of the condenser equipment and is then caught in an overhead accumulator drum. A portion of the distillate is recycled to the crude pot with the remainder being transferred to other refinery units.

One of the chief points of difficulty with respect to corrosion occurs in the area of the initial condensation of water that is carried over in the overhead line. The top temperature of the fractionating column is maintained above the boiling point of water. The initial condensate formed after the vapor leaves the column contains a high percentage of acidic materials such as hydrogen sulfide, hydrogen cyanide, CO HCl, etc. Due to the high concentration of acids dissolved in the water, the pH of the first condensate is quite low. For this reason the water is highly corrosive. It is important, therefore, that the first condensate be rendered less corrosive.

In the past, ammonia has been added at various points in the distillation circuit in an attempt to control the corrosiveness of condensed acidic materials. Ammonia, however, has not proven to be effective with respect to eliminating corrosion caused by the initial condensate. It is believed that ammonia has been ineffective for this purpose because it does not condense quickly enough to neutralize the acidic components of the first condensate. The ammonia tends to stay in the vapor phase until at least the point of the second condensation.

When using certain film-forming anticorrosive agents it has been found that a far more economical system is set up where the pH of the condensed liquids are maintained above about 4.5, and preferably at least about 5.0. This is true of virtually all amine film-forming inhibitors. A corrosion inhibitor of the film-forming type should be soluble in both aliphatics and aromatics in order to be dispersed throughout the stock. The inhibitors also should not tend to promote emulsification of the aqueous hydrocarbon phases.

The above problems have been substantially mitigated by using the teachings of Crawford, U.S. Pat. No. 3,447,89 l. The invention described in this patent comprises the discovery that the addition of a minor amount of morpholine to a crude oil charge or at various other points in the system effectively eliminates and/or controls corrosion that ordinarily occurs at and beyond the point of initial condensation of vapors within or leaving the distilling unit. The addition of morpholine to the crude substantially raises the pH of the initial condensate rendering the material noncorrosive or substantially less cor- GHQ-"CE:

N CHE-C Morpholine is relatively inexpensive and is effective at concentrations that are sufficiently low to make the process economically feasible. It is believed that morpholine is capable of neutralizing the acidic materials found in the first condensate primarily because it is readily condensed. For this reason it goes into the first condensate rather than being carried over to subsequent condensing units.

Morpholine can be added to the unit in any one of several places. First of all, morpholine can be added to the crude oil charge. This is a highly convenient method of carrying out the process since it will also neutralize condensate within the tower and in recirculating lines. The inhibitor can also be pumped directly into the gaseous overhead line. Morpholine can also be passed into the reflux line or can be added to recirculating H O at the top of the column. The particular point at which morpholine is added will depend largely on the design of the particular equipment, the personal preferences of the operator, and point where corrosion is most severe.

In many systems it is feasible to recirculate the water that condenses in the overhead system. In this particular operation a much lower quantity of morpholine is required to provide a highly satisfactory process. It has been found, for example, that the addition of as little as 4 ppm. of morpholine to crude oil stock based on the weight of the gross overhead provides a highly satisfactory system where the condensate water is recirculated. If the water is discarded rather than recirculated an increased amount of morpholine may be required to raise the pH of the first condensate above 4.5. The amount required can readily be determined by taking periodic pH readings or reading Corrosometer probes. The upper limit of the morpholine addition level depends largely on economic considerations. Unlike systems containing ammonia, it is not an essential that the pH be maintained below a given point. Morpholine as employed in the invention has no adverse effect on copper alloys and the like.

As was pointed out above, the use of morpholine to control the corrosiveness of the initial condensate lends itself well to the joint use of film-forming corrosion inhibitors. Such filmforming inhibitors operate most economically at a pH above 4.5. Due to the fact that morpholine is particularly effective in increasing the pH of the initial condensate the amount of film former that is required is substantially lessened.

Among the film-forming corrosion inhibitors which can be used in conjunction with morpholine to provide an overall system of protection are compounds formed by reacting certain aliphatic monoamines with polymerized fatty acids under salt-forming conditions.

The aliphatic monoamines used in preparing film-forming inhibitors are those amines having the general structural formula where R is an aliphatic hydrocarbon radical of eight to 22 carbon atoms in chain length and both R: and R are selected from the group consisting of hydrogen and an aliphatic hydrocarbon radical of one to 22 carbon atoms in chain length.

The above structural formula includes both primary and secondary aliphatic monoamines as well as the tertiary aliphatic monoamines. illustrative compounds coming within the above general formula include such primary amines as ndodecyl amine. n-tetradecyl amine, n-hexadecylamine, lauryl amine, myristyl amine, palmityl amine, stearyl amine. and pleyl amine. Other commercially available primary amines include coconut oil amine, tallow amine, hydrogenated tallow amine and cottonseed oil amine. Useful secondary amines are dilauryl amine, dimyristyl amine, dipalmityl amines. distearyl amine, dicoconut amine and dihydrogenated tallow amine. in the case of many of the above amines, it will be noted that the so$rce of alkyl substituent on the organic nitrogen is derived from a mixed vegetable oil or animal fat. For purposes of convenience, these compounds have been named from the derivative alkyl-containing components. This system of nomenclature, particularly in the case of alkyl substituents derived from naturally occurring products such as fats, oils and the like, is used for purposes of simplification. it is believed that those familiar with the art will readily understand that the alkyl substituent varies in the case of a coconut substituent with the alkyl groups containing from eight to 18 carbon atoms in chain length. Similarly, in the case of hydrogenated tallow. the alkyl substituent will vary from about 12 to 20 carbon atoms in chain length.

in addition to using primary or secondary amines as exemplified above, tertiary amines such as octyl dimethyl amine, pctadecyl dimethyl amine, octadecyl methyl benzyl amine, hexyl diethyl amine, trilauryl amine, tricoconut amine, tricapirylyl amine, and similar type compounds also may be used.

lPreferred aliphatic primary monoamines are amines having the general structural formula wherein R is an aliphatic hydrocarbon radical of from eight to 3.2 carbon atoms in chain length. A preferred material of this type is the commercial product Armeen SD sold by the Armour industrial Chemical Company which is known generically to the art as Soya amine. As applied to the above formula the R group is a mixed aliphatic radical which has the following components:

crcent 1H cxadecyl )ctadecyl 0 iDctadecenyl 5 ll)ctadecadicnyl t5 Out of the group of tertiary amines listed above one of the most effective is dimethyl hydrogenated tallow amine. This preferred species may be considered as an ammonium molecule which has had its three hydrogen atoms replaced by three alkyl groups. Two of these alkyl groups are methyl and the third is a mixed alkyl substituent derived from hydrogenated tallow.

A representative analysis of the mixed radicals of the hydrogenated tallow group IS as follows:

.i ercent M yristic 1 Palmitic Z9 Btearic it! Dleic One of the preferred commercial sources of this tertiary amine lS Armeen M HT" sold by Armour Industrial Chemical Company.

The polymerized fatty acids are well known and have been described in numerous publications. Excellent descriptions of these materials may be found in Industrial and Engineering Chemistry, 32, page 802 et seq. (1940), and in the text Fatty Acids" by Klare S. Markley, published by lnterscience Publishers, Inc., New York City, I947, pages 328 to 330. A specific example of such a polymer which has been found to be particularly useful is one which is prepared as a byproduct of the caustic fusion of castor oil in the manufacture of sebacic acid. This material is composed primarily of dicarboxylic acids derived by bimolecular addition in an olefinic polymerization where linkage occurs through the opening of at least two unsaturated bonds. Typical properties of a material so obtained are as follows:

acid value iaponification value Unsaponifiable matter.

percent 3.7 iodine No. 36 Moisture content, percent 0.86

The material is of course not pure but predominantly contains dicarboxylate polymers having about 34 to 36 atoms. A suitable commercial source of this dimer acid is Harchem Division of Wallace and Tiernan, Inc., and is known as "Century D-75 Acid.

A typical film-forming corrosion inhibitor useful in conjoint activity with morpholine may be prepared by combining 1 weight part of Armeen SD" with 2.57 weight parts of a polymerized fatty acid obtained as the residue of a dry distillation of castor oil with sodium hydroxide and reacting the mixture with stirring at a temperature of 60 C. for 20 minutes. The final reaction product is then dispersed in equal weight parts of a heavy aromatic solvent.

Another useful film-forming corrosion inhibitor composition is prepared by heating 14 parts of ArmeenM HT to the melting point and adding thereto 36 parts of Century D-75 Acid. The mixture was reacted for l0 minutes at l30-l 50 F. and the resultant product added to a heavy aromatic solvent in equal proportions by weight of product to solvent.

Distillation range mm. 760 initial boiling point C. 171 Percent:

to "C. I84 50 C. 230 )0 C. 260 End point C. 278

in reacting the above recited amines with polymerized fatty ,llCldS to obtain the film-forming compositions, care should be taken to maintain salt-forming conditions. This is done primarily by using reaction temperatures of from 25 to C, and by avoiding the presence of materials which cause the splitting out of water. This environment is sometimes referred to as neutralizing conditions". it is the salt producible from the above listed reactants which is of primary interest in the instant invention. Further care must be taken in conducting the reaction, to eliminate the possibility of presence of free amines in the final reaction product. Reaction proportions conductive to accomplishing this, typically include the above recited use of a weight ratio of typical polymer to typical monoamine of 2.57:1.

Additional film-forming compositions that can be used in conjunction with the subject inhibitor include those disclosed 111 U.S. Pat. No. 3,003,955 among others.

in practicing the invention set forth in U.S. Pat. No. 3,447,891 it is customary to determine the pH of the condensate after it has been subjected to heat exchange and the condensate is present in accumulators, knockout devices, and the like. It has been suspected that when the pH is determined at this point in the system that the true pH of the water in the overhead lines is not accurately measured. Hence the amount of inhibitor used to treat the condensate system is improperly dosed. In normal refinery operations it nearly has been impossible to determine the pH of the water entrained with the hydrocarbon vapors and acidic gases when these materials first leave a fractionating unit or a distillation column.

My invention allows a determination of the pH of the water contained in the condensate lines. The water vapor and acidic gases have been found to be much more corrosive due to the higher temperatures of the system at this particular point of sampling.

In order to determine the initial pH of the condensate as it leaves the distillation or fractionation power and thereby allowing correct proportioning of a neutralizing inhibitor to the system, I have found that the following apparatus and method to be extremely effective. In order to fully understand my apparatus and method reference may be had to the drawing.

THE DRAWING With particular reference to the drawing there is shown an overhead condensate line generally designated by the numeral ID. This line is fitted with a side stream feed line 12 which feeds a small portion of the hydrocarbon vapor, acidic gas and water vapor to a horizontally mounted heat exchanger 14 which is generally in the form of a cylindrical tank having sidewalls 16, a front end 18 and a back cover 20.

Positioned within the heat exchanger 14 are a series of hairpin-shaped tubes 24 whose inlet and outlet openings are mounted and positioned on tube bundle wall 26 which is mounted in perpendicular relationship to sidewall 16 of the heat exchanger 14. The heat exchanger is fitted with a hydrocarbon vapor inlet 28 which receives hydrocarbon vapor from line 12. Hydrocarbon vapor enters the heat exchanger at hydrocarbon vapor inlet 28 and fiows parallel to the tubes 24. Parallel flow of the vapor to the heat exchanger is further regulated by means of a bafi'le 30a which is an elongated horizontally disposed plate which is perpendicularly affixed to tube bundle wall 26.

The hydrocarbon vapor which has been cooled by the tubes 24 is discharged from the heat exchanger through vapor discharge outlet 30 where it is transported through line 32 to the condensate line 10.

Fitted along the bottom wall 16 of the heat exchanger and in communication with the interior of the heat exchanger is a water condensate sample receiver 34 which is shown in the drawing as being a cup-shaped device whose bottom is fitted with a condensate sample withdrawal line 36 and the valve 38 for such line. Positioned within the sample receiver 34 is a thermometer 35 whose sensing element contacts the cooled hydrocarbon vapors. The space in the heat exchanger defined between front end 18 and tube bundle wall 26 comprises a cooling media chamber designated generally by the numeral 40. The chamber is divided into an inlet compartment 42 and an outlet compartment 44 which are divided by a horizontally disposed partition 46. Cooling media enters inlet compartment 42 through line 48 which is fitted with valve 50. The cooling liquid then passes through the tubes 24 and into outlet compartment 44 and is discharged through line 52 which contains valve 54.

For feeding neutralizing inhibitor such as morpholine to the overhead condensate line there is provided a storage tank 56 and a feedline 58 which communicates with the overhead condensate line 10. Line 58 is fitted with proportioning pump 60 for feeding incrementally or continuously neutralizing inhibitor contained in storage tank 56.

In operation hydrocarbon vapor passing through the heat exchanger is cooled to a temperature at which water is first condensed as a liquid. The water is then collected in the water condensate sample receiver 34 where it is withdrawn through line 36 to a sample container in which the pH of the water may be determined. The temperature at which the water condenses in the heat exchanger is regulated by controlling the amount of cooling liquid entering line 48 or being discharged from line 52. This regulation of flow is adjusted by either/or valves 50 or 54. Once the pH of the condensate has been determined the amount of morpholine or other inhibitor used to adjust the pH of the condensate may be determined and the amount then fed to the overhead condensate line 50 from storage tank 56.

Experimentation with the device thus described in the drawing has shown that the corrosivity as indicated by the pH of the condensate is most severe at the highest temperature at which condensation occurs.

It is important, therefore, to feed the neutralizing inhibitor an amount sufficient to adequately neutralize the pH of the condensate when such condensate is formed at the maximum temperature in the condensate line. By thus treating the system it is possible to optimize effectiveness of morpholine either alone or in conjunction with said film-forming inhibitors ofthe type described in US. Pat. No. 3,447,89l.

THE PROCESS OF THE INVENTION Based upon the above described techniques the invention provides a method for optimizing corrosion protection of distilling units containing a fractionating tower and overhead lines used in the distillation treatment of petroleum production where corrosion is occasioned by acidic compounds dissolved in the water of initial condensate which comprises the steps of:

a. Withdrawing a small side stream sample of product from the overhead line;

b. Subjecting said sample to a heat exchanger step whereby the sample is cooled to a temperature whereby water is condensed from the sample;

0. Collecting at least a portion of the condensed water removed from the sample;

d. Determining the pH of said water; and then e. Adding a neutralizing inhibitor to the overhead line in an amount sufficient to adjust the pH of the condensate to at least 4.0.

CONCLUSIONS The invention provides a technique for maximizing the effectiveness of neutralizing type inhibitors exemplified by the well-known chemical, morpholine. It allows the determination of the corrosivity of condensate with a high degree of accuracy not heretofore obtainable by the measurement of pH by the use of accumulators, knockout devices, and the like. The term initial condensate as it is used herein signifies a phase formed when the temperature of the surrounding environment reaches the dew point of water. At this point a mixed phase of liquid water, hydrocarbon and vapor may be present. As is evident from the above discussion such initial condensate may occur within the distilling unit itself or in subsequent conductors.

It is understood that many obvious variations of my invention may be made without departing from the basic concepts set forth herein. For example, the condensate collected in the water condensate sample receiver 34 may be continuously monitored on a pH meter as well as the temperature of the vapor inside of the heat exchanger may be continually determined by means of a recording thermometer. When this technique is employed the pH meter or thermometer may be used to actuate the pump 60 for monitoring the feed of the neutralizing inhibitor from storage tank 56.

Having thus described the invention I claim:

1. A process for inhibiting corrosion during the distillation of a petroleum product in a distilling unit containing a fractionating tower and an overhead line caused by acidic compounds dissolved in the water of the initial condensating distilling petroleum product which comprises the steps of:

a. Withdrawing a small side stream sample of product from the overhead line at a point prior to the formation of an initial condensate;

b. Subjecting the sample to a heat exchange step to cool the sample to a temperature corresponding to the dew point of the water in said sample, thereby producing a simulated initial condensate;

c. Collecting at least a portion of the condensed water removed from the sample;

d. Determining the pH of said water; and then e. Adding a neutralizing inhibitor to the overhead line in an amount sufficient to adjust the pH of the initial condensate to at least 4.0.

2. The process of claim 1 where the small side stream sample is continuously withdrawn from the overhead line and the l0l027 M 0 neutralizing corrosion inhibitor is continuously added to the overhead line.

The process of claim 1 where the neutralizing corrosion inhibitor is morpholine.

l. The method of claim 1 where the neutralizing corrosion 5 inhibitor comprises morpholine and a film-forming amine.

W 3 I l 

2. The process of claim 1 where the small side stream sample is continuously withdrawn from the overhead line and the neutralizing corrosion inhibitor is continuously added to the overhead line.
 3. The process of claim 1 where the neutralizing corrosion inhibitor is morpholine.
 4. The method of claim 1 where the neutralizing corrosion inhibitor comprises morpholine and a film-forming amine. 